Curable resin composition and flexographic plate material using the same

ABSTRACT

The invention provides a curable resin composition having an addition polymerization-based block copolymer (I) and an ethylenic unsaturated compound (II). Also provided is a flexographic plate material using the curable resin composition as its constituent. The flexographic plate material containing the curable resin composition of the present invention, can be cured to form strong and extendable areas and can be used to make flexographic plates that can form a sharp image plate surface even for a fine image. The flexographic plate material of the present invention is particularly useful in printing on cardboards, recycled paper or other paper articles with rough surfaces.

TECHNICAL FIELD

The present invention relates to a curable resin composition and aflexographic plate material that contains the curable resin compositionas its constituent. Containing the curable resin composition as itsconstituent, the flexographic plate material of the present invention ishighly strong and extendable in the cured area and can thus be used tomake flexographic plates that can form a sharp image plate surface evenfor a fine image. The flexographic plate material of the presentinvention is particularly suitable for printing on cardboard, recycledpaper, or other materials having a rough surface.

BACKGROUND ART

A flexographic plate is a type of relief printing plates and generallyincludes an elastic relief plate made of rubber or photosensitive resin,to which a liquid ink is applied for printing. Flexographic plates canprint on rough or curved surfaces and are thus widely used for printingimages on wrapping, magazines, cardboards, labels, and bottles.Flexographic plates were previously manufactured by pouring moltenrubber in a mold and then curing the rubber or by manually carving arubber plate. Neither of these techniques was suitable for producingaccurate flexographic plates, however. Lately, the development of a newtechnique that uses curable resins to make flexographic plate materialshas made the production of flexographic plates considerably simple.

A typical flexographic plate material of the newly developed typeincludes, from top to bottom, a surface protective layer; a layer of acurable resin composition that is curable by irradiating with an activeenergy ray and is composed of an elastomer, such as urethane rubber,butyl rubber, silicon rubber and ethylene propylene rubber, an ethylenicunsaturated compound and, if necessary, a photopolymerization initiator;an adhesive layer; and a substrate (See, for example, “Kankosei jushi nokiso to jitsuyo (Basics and applications of photosensitive resins)”Supervised by Kiyoshi Akamatsu, CMC Co. Ltd. (2001) 152-160).

In one process for producing a flexographic plate from such aflexographic plate material, a film carrying a negative image of aletter, diagram, picture, pattern, or any other image to be printed isfirst applied to the surface of the protective film opposite to thesubstrate. The negative film is then irradiated with an active energyray from above, so that the predetermined areas of the curable resincomposition layer are selectively cured by the action of the activeenergy ray transmitted through the imaged area of the film and becomeinsoluble to solvent. Subsequently, the negative film and the protectivefilm are removed and a solvent is applied to remove the non-irradiatedor uncured areas of the curable resin composition layer (developmentstep) and thereby form an image area (i.e., image plate surface). Thiscompletes a flexographic plate (See, for example, “Kankosei jushi nokiso to jitsuyo (Basics and applications of photosensitive resins)”Supervised by Kiyoshi Akamatsu, CMC Co. Ltd. (2001) 152-160; JapanesePatent Publication No. S55-34415; U.S. Pat. No. 4,323,636; JapanesePatent Publication No. S51-43374; and Japanese Patent ApplicationLaid-Open No. H2-108632).

In an effort to ensure formation of fine dots and lines on theflexographic plates and prevent chipping of the image plate surfaceduring development, improvements have been made as to the type andproportion of the resin to be added to the curable resin composition.One example involves the use of a styrene-based block copolymer in whichthe part of the copolymer formed of a conjugated diene has a significantbound vinyl content (See, Japanese Patent Application Laid-Open No.H5-134410). In another example, a certain thermoplastic elastomercomposed of a monovinyl-substituted aromatic hydrocarbon and aconjugated diene is used in conjunction with a diene-based liquid rubberthat has a high average proportion of bound vinyl units (See, JapanesePatent Application Laid-Open No. 2000-155418).

The technique described in Japanese Patent Publication No. S55-34415employs crystalline 1,2-polybutadiene in conjunction with a polymercompound, such as polyisoprene rubber, that comprises as itsconstituents at least one of ethylene, butadiene and isoprene. Adrawback of this technique is that the uncrosslinked rubber used in theresin makes the flexographic plate susceptible to deformation (coldflow) during storage or transportation of uncured plates. U.S. Pat. No.4,323,636 and Japanese Patent Publication No. S51-43374 describes theuse of a certain block copolymer (preferably a styrene-isoprene-styrenetriblock copolymer or a styrene-butadiene-styrene triblock copolymerhaving a particular composition) that is a thermoplastic elastomer andin which the hard segments have a grass transition temperature of 25° C.or above. In these techniques, the part of the copolymer formed ofpolystyrene causes a cohesive force, which reduces deformation ofuncured plates. However, the polystyrene blocks of the elastomer do notundergo crosslinking even when irradiated with an active energy ray, sothat the uncrosslinked polystyrene blocks causes a poor solventresistance of the cured area. Consequently, the image area tends toswell when a solvent is applied to remove the uncured area, resulting ininsufficient reproducibility and poor ink resistance of the flexographicplates. Another flaw of this technique is that the strength andextension of the cured area are insufficient especially for forming afine image area and the resulting flexographic plate becomes lessdurable. This causes chipping in the edge of the image plate surfaceduring the removal of the uncured area by washing with a solvent and,when necessary, a brush. As a result, the desired sharp image platesurface may not be obtained.

In the technique described in Japanese Patent Laid-Open Publication No.H2-108632, the flexibility of flexographic plates is increased by theuse of a binder (preferably, a styrene-butadiene-styrene triblockcopolymer) containing thermoplastic and elastomeric domains, incombination with a particular addition polymerizable ethylenicunsaturated monomer. Despite its improved flexibility, the resinaccording to this technique includes some part formed of a polystyreneblock similar to the one described above, which makes the flexographicplate less resistant to solvent. The cured area of the flexographicplate obtained by this technique is not strong enough to form a fineimage area.

Although both of the techniques described in Japanese Patent ApplicationLaid-Open No. H5-134410 and No. 2000-155418 have managed to improve thecurability of the part of the styrene-based thermoplastic elastomerformed of conjugated diene units and have managed to increase thetoughness of the resulting flexographic plate, the similar polystyreneblock part present in the thermoplastic elastomers suppresses thesolvent resistance and the flexographic plates are not operative enoughto form a fine image area.

Accordingly, it is an object of the present invention to provide acurable resin composition suitable for the production of a flexographicplate material that allows printing on an article with rough surfaces,such as cardboard and recycled paper. The curable composition of thepresent invention can be cured to form strong and extendable areas andcan thus be used to make flexographic plates that can form a sharp imageplate surface even for a fine image. It is also an objective of thepresent invention to provide a flexographic plate material that uses thecurable resin composition as its constituent.

DISCLOSURE OF THE INVENTION

The present invention achieves this object by providing the followingcompositions or material:

<1> A curable resin composition, containing an additionpolymerization-based block copolymer (I) and an ethylenic unsaturatedcompound (II), wherein:

the addition polymerization-based block copolymer (I) is selected fromblock copolymers comprising at least one polymer block A and at leastone polymer block B, and the hydrogenated products thereof;

the polymer block A essentially comprises an aromatic vinyl compoundunit containing at least 1% by mass of an alkylstyrene-derivedstructural unit (a) (which may be referred to simply as “structural unit(a),” hereinafter) in which at least one alkyl group having 1 to 8carbon atoms is bound to a benzene ring; the polymer block B essentiallycomprises a conjugated diene compound unit; and

at least the moiety of polymer block A can undergo crosslinking uponexposure to an active energy ray.

<2> The curable resin composition according to <1> above, furthercontaining a photopolymerization initiator (III).

<3> The curable resin composition according to <1> or <2> above, furthercontaining a softener (IV).

<4> The curable resin composition according to any one of <1> to <3>above, wherein the structural unit (a) is a p-methylstyrene unit.

<5> A flexographic plate material, using the curable resin compositionaccording to any one of <1> to <4> above.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention will now be described in further detail.

The addition polymerization-based block copolymer (I), the essentialcomponent of the curable resin composition of the present invention, isselected from block copolymers comprising at least one polymer block Aand at least one polymer block B, and the hydrogenated products thereof.The polymer block A essentially comprises an aromatic vinyl compoundunit that contains at least 1% by mass of an alkylstyrene-derivedstructural unit (a) in which at least one alkyl group having 1 to 8carbon atoms is bound to a benzene ring. The polymer block B essentiallycomprises a conjugated diene compound unit. At least the moiety ofpolymer block A can undergo crosslinking upon exposure to an activeenergy ray.

Examples of alkylstyrenes for forming the structural unit (a) of thepolymer block A include o-alkylstyrene, m-alkylstyrene, p-alkylstyrene,2,4-dialkylstyrene, 3,5-dialkylstyrene, and 2,4,6-trialkylstyrene withtheir alkyl groups having 1 to 8 carbon atoms, and halogenatedalkylstyrenes in which one or more of the hydrogen atoms borne by thealkyl groups of the alkylstyrenes have been substituted with halogenatoms. Specific examples of the alkylstyrenes for forming the structuralunit (a) include o-methylstyrene, m-methylstyrene, p-methylstyrene,2,4-dimethylstyrene, 3,5-dimethylstyrene, 2,4,6-trimethylstyrene,o-ethylstyrene, m-ethylstyrene, p-ethylstyrene, 2,4-diethylstyrene,3,5-diethylstyrene, 2,4,6-triethylstyrene, o-propylstyrene,m-propylstyrene, p-propylstyrene, 2,4-dipropylstyrene,3,5-dipropylstyrene, 2,4,6-tripropylstyrene, 2-methyl-4-ethylstyrene,3-methyl-5-ethylstyrene, o-chloromethylstyrene, m-chloromethylstyrene,p-chloromethylstyrene, 2,4-bis(chloromethyl)styrene,3,5-bis(chloromethyl)styrene, 2,4,6-tri(chloromethyl)styrene,o-dichloromethylstyrene, m-dichloromethylstyrene, andp-dichloromethylstyrene.

The polymer block A may contain one or more units of the above-describedalkylstyrenes and halogenated alkylstyrenes for forming the structuralunit (a). Of these, p-methylstyrene unit, which can readily undergocrosslinking and is readily available, is particularly preferred as thestructural unit (a).

The polymer block A contains aromatic vinyl compound units other thanthose forming the structural unit (a). Examples of such structural unitsinclude those formed of styrene, α-methylstyrene, β-methylstyrene,monofluorostyrene, difluorostyrene, monochlorostyrene, dichlorostyrene,methoxystyrene, vinylnaphthalene, vinylanthracene, indene, andacetonaphthylene. Of these, styrene and α-methylstyrene are particularlypreferred.

The polymer block A of the addition polymerization-based block copolymer(I) forms hard segments. The alkyl groups, which are each bound to abenzene ring to form the structural unit (a), serve to introduce crosslinkages in the hard segments of the polymer block A as they undergo thestatic crosslinking reaction upon exposure to an active energy ray.

The proportion of the structural unit (a) in the polymer block A is 1%by mass or more, preferably 10% by mass or more, and more preferably 40%by mass or more with respect to the mass of the polymer block A. Thepolymer block A may be made entirely of the structural unit (a). If theproportion of the structural unit (a) is less than 1% by mass, then thecross linkages are hardly introduced into the polymer block A, resultingin insufficient curability of the resulting curable resin composition.In the polymer block A, the structural unit (a) and other aromatic vinylcompound units may be linked to one another either randomly, in blocksor in tapered blocks.

Preferably, the polymer block A is present in the additionpolymerization-based block copolymer (I) in an amount of 10 to 40% bymass. If the amount of the polymer block A is less than 10% by mass,then the ability of the polymer block A to physically aggregate to formthe hard segments of the addition polymerization-based block copolymer(I) becomes weak. As a result, the uncured plate (i.e., flexographicplate material yet to be irradiated with an active energy ray) formed ofthe curable resin composition containing the additionpolymerization-based block copolymer (I) becomes susceptible to coldflow. This causes significant deformation of the plate during storage ortransportation, making the flexographic plate less suitable forprinting. If the amount of the polymer block A is greater than 40% bymass, then the rubber elasticity of the curable resin composition isdecreased, making it difficult for the flexographic plate to effectivelytransfer an ink to cardboards, recycled paper, and other paper articlesthat have rough surfaces.

When necessary, the polymer block A of the addition polymerization-basedblock copolymer (I) may include, along with the structural unit composedof the aromatic vinyl compound containing the structural unit (a), asmall amount of structural units composed of other polymerizablemonomers. The proportion of the structural unit composed of such otherpolymerizable monomers is preferably 30% by mass or less, and morepreferably 10% by mass or less based on the mass of the polymer block A(or the total mass of the polymer blocks A when the additionpolymerization-based copolymer (I) contains two or more polymer blocksA). Examples of the other polymerizable monomers include 1-butene,pentene, hexene, butadiene, isoprene, and methyl vinyl ether.

Aside from the polymer block A composed of the aromatic vinyl compoundunit containing the structural unit (a), the additionpolymerization-based block copolymer (I) for use in the presentinvention may contain a polymer block composed of an aromatic vinylcompound that does not contain the structural unit (a).

The polymer block B of the addition polymerization-based copolymer (I)is composed essentially of a conjugated diene compound unit. Examples ofthe conjugated diene compounds include butadiene, isoprene,2,3-dimethyl-1,3-butadiene, 1,3-pentadiene, and 1,3-hexadiene. Thepolymer block B may be composed solely of one of these conjugated dienecompounds, or it may be composed of two or more of the conjugated dienecompounds. The polymer block B is preferably composed of butadiene,isoprene, or a mixture of butadiene and isoprene. While the type and theamount of the microscopic structure for forming the polymer block B arenot limited to a particular type and amount, preferably 5 to 90 mol %,more preferably, 20 to 70 mol % of the monomer units of the polymerblock B are linked by 1,2-linkage when the polymer block B is composedof, for example, butadiene units alone and when the polymer block B iscomposed of isoprene units alone or combination of butadiene units andisoprene units, 5 to 80 mol %, preferably 10 to 60 mol % of the monomerunits are linked by 1,2-linkage or 3,4-linkage. When two or moreconjugated dienes are used, the monomer units may be linked to oneanother either randomly, in blocks, in tapered blocks, or by two or moreof these manners of linking.

When necessary, the polymer block B may contain, along with thestructural units composed of the conjugated diene compounds, a smallproportion of structural units composed of other polymerizable monomers.The proportion of such other polymerizable monomers is preferably 30% bymass or less and, more preferably, 10% by mass or less based on the massof the polymer block B that forms the addition polymerization-basedblock copolymer (1) (or the total mass of the polymer blocks B when theaddition polymerization-based copolymer (I) contains two or more polymerblocks B). Examples of the other polymerizable monomers include styrene,α-methylstyrene, and the aforementioned alkylstyrenes for forming thestructural unit (a) (preferably, p-methylstyrene).

It is particularly preferred that the polymer block B be one of thefollowings: a polyisoprene block composed of isoprene units or ahydrogenated polyisoprene block in which some or all of thecarbon-carbon double bonds originating from the isoprene units have beenhydrogenated; a polybutadiene block composed of butadiene units or ahydrogenated polybutadiene block in which some or all of thecarbon-carbon double bonds originating from the butadiene units havebeen hydrogenated; or a copolymer block composed of isoprene units andbutadiene units or a hydrogenated copolymer block in which some or allof the carbon-carbon double bonds originating from the isoprene units orthe butadiene units have been hydrogenated.

As far as the polymer block A and the polymer block B are linked to oneanother, they may be linked in any manner of linking, forming astraight-chained, branched or radial molecule of the additionpolymerization-based block copolymer (I). Two or more of these mannersof linking may be combined in one molecule. Preferably, the polymerblock A and the polymer block B are linked together to form astraight-chained molecule. Examples of the straight-chained moleculesinclude triblock copolymers as denoted by A-B-A, tetrablock copolymersas denoted by A-B-A-B and pentablock copolymers as denoted by A-B-A-B-A,given that “A” represents the polymer block A and “B” represents thepolymer block B. Of these, triblock copolymers (“A-B-A”) areparticularly preferred because of the flexibility and readiness of theproduction of the addition polymerization-based block copolymer (I).

While the addition polymerization-based copolymer (I) may have anynumber average molecular weight, it preferably has a number averagemolecular weight in the range of 30000 to 1000000, and more preferablyin the range of 40000 to 300000. The term “number average molecularweight” as used herein refers to a number average molecular weight asdetermined by gel permeation chromatography (GPC) using polystyrenestandards.

The addition polymerization-based copolymer (I) of the present inventioncan be produced, for example, by a known anionic polymerizationtechnique. Specifically, the alkylstyrene for forming the structuralunit (a), or a mixture of the alkylstyrene for forming the structuralunit (a) and the aromatic vinyl compound, and the conjugated dienecompound are sequentially polymerized to form a block copolymer (i.e.,non-hydrogenated form of the addition polymerization-based blockcopolymer (I)). Using an initiator such as an alkyllithium compound, thereaction is carried out in n-hexane, cyclohexane, or other organicsolvents that are inert to the polymerization.

When necessary, the resulting block copolymer is hydrogenated. Thehydrogenation reaction is generally carried out in a saturatedhydrocarbon solvent such as cyclohexane at a reaction temperature of 20to 100° C. under a hydrogen pressure of 0.1 to 10 MPa and in thepresence of a hydrogenation catalyst, giving a hydrogenated product ofthe addition polymerization-based block copolymer (I). Examples of suchhydrogenation catalysts include Raney nickels; heterogeneous catalystscontaining metals, such as Pt, Pd, Ru, Rh, and Ni, carried by carbon,alumina, diatomite, and other suitable carriers; Ziegler catalystscontaining an organic metal compound of, for example, cobalt, nickel andother group 9 or group 10 metals, combined with an organoaluminumcompound or organolithium compound, such as triethylaluminum andtriisobutylaluminum; and metallocene catalysts containing abis(cyclopentadienyl) compound of transition metals, such as titanium,zirconium, and hafnium, combined with an organic metal compound, such aslithium, sodium, potassium, aluminum, zinc, and magnesium.

While the degree of hydrogenation may be adjusted depending on whatphysical properties are required of the curable resin composition of thepresent invention, it is preferred that 70% of more, preferably 85% ormore, and more preferably 95% or more of the carbon-carbon double bondsthat result from the conjugated diene compound units of the polymerblock B for forming the addition polymerization-based block copolymer(I) are hydrogenated when heat resistance, weather resistance, and ozoneresistance are particularly important.

The degree of hydrogenation of the carbon-carbon double bonds thatresult from the conjugated diene compound units of the polymer block Bcan be determined by measuring the amount of the carbon-carbon doublebonds in the polymer block B before the hydrogenation reaction and theamount after the hydrogenation reaction by means of iodimetry, IRspectrophotometry, nuclear magnetic resonance or other suitabletechniques and taking the difference between these amounts.

Preferably, the addition polymerization-based block copolymer (I) ispresent in the curable resin composition of the present invention in anamount of 30 to 99% by mass and, more preferably, in an amount of 50 to95% by mass. If the amount of the addition polymerization-based blockcopolymer (I) in the curable resin composition is less than 30% by mass,then the resulting uncured plate (i.e., flexographic plate material yetto be irradiated with an active energy ray), formed of the curable resincomposition, will not be hard enough, so that the plate becomessusceptible to cold flow and, thus, deformation during its storage ortransportation. Such flexographic plates are less suitable for printing.If the amount is more than 99% by mass, then the resulting curable resincomposition becomes excessively hard, making it difficult for theflexographic plate to effectively transfer an ink to cardboards,recycled paper and other paper articles that have rough surfaces.

The ethylenic unsaturated compound (II) for use in the curable resincomposition of the present invention may be a carboxylic acid havingcarbon-carbon double bonds, such as acrylic acid, methacrylic acid,fumaric acid, and maleic acid, or an ester thereof (for example, diethylfumarate, dibutyl fumarate, dioctyl fumarate, distearyl fumarate,butyloctyl fumarate, diphenyl fumarate, dibenzyl fumarate, dibutylmaleate, dioctyl maleate, bis(3-phenylpropyl) fumarate, dilaurylfumarate, and dibehenyl fumarate); (meth)acrylamide, such as acrylamide,methacrylamide, and diacetone acrylamide; an N-substituted maleimide,such as N-n-hexylmaleimide, N-cyclohexylmaleimide, N-n-octylmaleimide,N-2-ethylhexylmaleimide, N-n-decylmaleimide, and N-n-laurylmaleimide; adi(meth)acrylate, such as ethylene glycol di(meth)acrylate, diethyleneglycol di(meth)acrylate, propylene glycol di(meth)acrylate, dipropyleneglycol di(meth)acrylate, polyethylene glycol di(meth)acrylate,polypropylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate,1,6-hexanediol di(meth)acrylate, and 1,9-nonanediol di(meth)acrylate;styrene, vinyl toluene, divinyl benzene, diallyl phthalate, and triallylcyanulate. These compounds may be used either individually or incombination of two or more compounds.

Preferably, the ethylenic unsaturated compound (II) is present in thecurable resin composition of the present invention in an amount of 1 to70% by mass and, more preferably, in an amount of 5 to 50% by mass. Ifthe amount of the ethylenic unsaturated compound (II) present in thecurable resin composition is less than 1% by mass, then the resultinguncured plate (i.e., flexographic plate material yet to be irradiatedwith an active energy ray) made of the curable resin composition may notcure to a sufficient degree upon irradiation with an active energy ray.As a result, desired sharp image plate surfaces may not be obtained. Ifthe amount is more than 70% by mass, then the curable resin composition,once cured, becomes excessively hard, making it difficult for theflexographic plate to effectively transfer an ink to cardboards,recycled paper, and other paper articles that have rough surfaces.

The photopolymerization initiator (III), an optional component of thecurable resin composition of the present invention, may be benzophenone,benzoin, benzoin methyl ether, benzoin ethyl ether, benzoin isopropylether, α-methylolbenzoin, α-methylolbenzoin methyl ether,α-methoxybenzoin methyl ether, benzoin phenyl ether, andα-t-butylbenzoin. These compounds may be used either individually or incombination of two or more compounds.

When used, the photopolymerization initiator (III) is added to thecurable resin composition of the present invention preferably in anamount of 0.1 to 10% by mass and, more preferably, in an amount of 0.2to 8% by mass with respect to the total amount of the curable resincomposition.

If the amount of the photopolymerization initiator (III) is less than0.1% by weight, then the curable resin composition does not undergocrosslinking to a sufficient degree, resulting in that it is hard toobtain sufficient curability. In comparison, if contained in amountslarger than 10% by mass, the photopolymerization initiator (III) leadsto a reduced transmittance of the curable resin composition to an activeenergy ray and, thus, a reduced sensitivity to irradiation. As a result,it is hard to form sufficient crosslinking.

The softener (IV) is another optional component of the curable resincomposition of the present invention. Examples of such softeners includediene-based liquid rubbers, such as liquid polyisoprene, liquid1,2-polybutadiene, liquid 1,4-polybutadiene, liquid poly 1,2-pentadiene,liquid ethylene-butadiene copolymer, liquid acrylonitrile-butadienecopolymer, and modified products and hydrogenated products thereof;petroleum-based softeners, such as paraffin-, naphthene-, and aromaticcompound-based processed oils; liquid paraffin; and vegetable oil-basedsofteners, such as peanut oil and rosin. These softeners may be usedeither individually or as a mixture of two or more softeners. Of these,diene-based liquid rubber is particularly preferred for use in thepresent invention.

When used, the softener (IV) may be added in any amount that does notaffect the objective of the present invention. The softener, however, ispreferably added in an amount of 5 to 50% by mass with respect to thetotal amount of the curable resin composition.

As far as the objective of the present invention is not interfered, thefollowing polymers may further be added to the curable resin compositionof the present invention: rubbers, such as natural rubbers, syntheticpolyisoprene rubber, polybutadiene rubber, styrene-butadiene rubber,chloroprene rubber, ethylene-propylene rubber, acryl rubber, butylrubber, and acrylonitrile-butadiene rubber; and styrene-based blockcopolymers, such as polystyrene-polyisoprene-polystyrene blockcopolymers, polystyrene-polybutadiene-polystyrene block copolymers, andhydrogenated products thereof. These polymers may be used eitherindividually or in combination of two or more polymers.

If necessary, various auxiliary additives used in common photosensitiveresin compositions may be added to the curable resin composition of thepresent invention, including heat polymerization inhibitors, such as2,6-di-t-butyl-p-cresol, p-methoxyphenol, pentaerythritoltetrakis[3-(3′,5′-di-t-butyl-4′-hydroxy)phenylpropionate], hydroquinone,t-butyl catechol, t-butyl hydroxyanisole, and 4,4′-butylidenebis(3-methyl-6-t-butyl)phenol; UV absorbents; antihalation agents; andphotostabilizers. These compounds may be used either individually or incombination of two or more.

The curable resin composition of the present invention can be producedby mixing/kneading the above-described addition polymerization-basedblock copolymer (I), the ethylenic unsaturated compound (II), and, ifnecessary, the photopolymerization initiator (III), the softener (IV),and the other optional additives on, for example, a kneader.

The active energy ray that is used to cure the curable resin compositionof the present invention may be a particle beam, electromagnetic wave,or a combination of these. Examples of the particle beam includeelectron beam (EB) and α-ray, and examples of the electromagnetic waveinclude ultraviolet ray (UV), visible light, infrared ray, γ-ray, andX-ray. Of these, electron beam (EB) and ultraviolet ray (UV) areparticularly preferred. These active energy rays can be irradiated usingknown apparatuses. The electron beam may be accelerated at a voltage of0.1 to 10 MeV and irradiated at a dose of 1 to 500 kGy. A lamp with anirradiation wavelength of 200 to 450 nm may be preferably used as thesource of ultraviolet ray (UV). Examples of the electron beam (EB)source include tungsten filament, and examples of the ultraviolet ray(UV) source include low-pressure mercury-vapor lamp, high-pressuremercury-vapor lamp, ultraviolet mercury lamp, carbon arc lamp, xenonlamp, and zirconium lamp.

The curable resin composition of the present invention is particularlysuitable for use as a constituent of flexographic plates. Specifically,the use of the curable resin composition of the present invention as aconstituent can allow production of flexographic plates that undergominimum deformation (cold flow) during storage or transportation ofuncured plates (i.e., flexographic plate material yet to be irradiatedwith the active energy ray), are ideal for printing sharp images, andthat can ensure high quality printing in printing on cardboards,recycled paper, and other poor quality paper articles having roughsurfaces by being capable of effectively transferring an ink onto suchpaper articles.

A preferred technique for producing a flexographic plate material thatuses the curable resin composition of the present invention as aconstituent involves molding the molten curable resin composition of thepresent invention into a desired shape by using press molding, extrusionmolding, or calendering. The composition is molded such that it isdeposited on a substrate to a thickness of approximately 200 μm to 20mm. The substrate may be a plastic sheet, rubber sheet, foamed olefinsheet, foamed rubber sheet, foamed urethane sheet, or metal sheet. Ifnecessary, an adhesive may be used to adhere the substrate to thecurable resin composition of the present invention. Also, if necessary,a protective film, such as a polyethylene terephthalate film, may beapplied to the surface of the curable resin composition so as to keepthe curable resin composition of the present invention from beingaffected by oxygen present in the atmosphere.

One exemplary technique for obtaining a flexographic plate from theflexographic plate material using the curable resin composition of thepresent invention as a constituent includes the following procedures:First, the surface protective film, if any, is removed. A film carryinga negative image of a letter, diagram, picture, pattern or any otherimage to be printed is then applied to the layer of the curable resincomposition of the present invention. The negative film is irradiatedwith an active energy ray from above, so that the predetermined area ofthe curable resin composition layer is selectively cured by the actionof the active energy ray transmitted through the imaged area of thenegative film and become insoluble to solvent. Subsequently, thenegative film is removed and a solvent is applied to remove thenon-irradiated or uncured area of the curable resin composition layerand thereby form an image area.

Examples of the solvents that can be used to remove the uncured areainclude tetrachloroethylene, aromatic hydrocarbons, such as, toluene andxylene, acetates, limonene, and decahydronaphthalene, as well asmixtures of these solvents with n-butanol, 1-pentanol, and benzylalcohol. The unexposed area (i.e., uncured area of the curable resincomposition) may be dissolved away, for example, by applying the solventsprayed from a nozzle or by washing with the solvent and a brush.

The flexographic plate material obtained by using the curable resincomposition of the present invention as a constituent undergoes minimumdeformation during storage or transportation of uncured plates (i.e.,flexographic plate material yet to be irradiated with the active energyray). The flexographic plate material of the present invention can forma sharp image plate surface by irradiating with the active energy ray,and can ensure high quality printing in printing on cardboards, recycledpaper, and other poor quality paper articles with rough surfaces bybeing capable of effectively transferring an ink onto such paperarticles.

The present invention thus provides a curable resin composition suitablefor the production of a flexographic plate material that allows printingon an article with rough surfaces, such as cardboard and recycled paper.The curable composition of the present invention can be cured to formstrong and extendable areas and can thus be used to make flexographicplates that can form a sharp image plate surface even for a fine image.The present invention also provides a flexographic plate material thatuses the curable resin composition as its constituent.

The present invention will now be described in further detail withreference to Examples, which are not intended to limit the scope of theinvention in any way. In each of Examples and Comparative Examplesdescribed below, an exemplary curable resin composition was evaluatedfor different physical properties. The evaluation was made according tothe following methods.

(1) The Shape Stability of Uncured Plate Prior to Irradiation withActive Energy Ray

A 2 mm-thick sheet made of each of the curable resin compositionsobtained in Examples and Comparative Examples was cut into a 5 cm long×5cm wide sample piece. While a 30 g/cm² load was applied to the entiresurface of the sample piece, the sample piece was left in a 40° C.atmosphere for 24 hours. Subsequently, the thickness of the sample piecewas measured and the sample was determined to be “acceptable” if thedecrease in thickness was less than 2% (indicated by a circle) and “notacceptable” if the decrease in thickness was 2% or more (indicated by across).

(2) The Tensile Strength at Break and Elongation at Break After Curing

A 2 mm-thick sheet made of each of the curable resin compositionsobtained in Examples and Comparative Examples was entirely irradiatedwith a UV ray at 30 mW/cm² (radiation wavelength=200-450 nm) for 1minute. Following this, a No. 5 dumbbell-shaped sample piece asspecified by JIS K 6251 was made out of the sheet and was stretched onan INSTRON universal tester at a rate of 500 mm/min at 23° C. todetermine the tensile strength at break (MPa) and the elongation atbreak (%).

(3) Reproducibility of Negative Image

A 15 cm×15 cm sample piece was cut out from a 2 mm-thick pressed sheetmade of each of the curable resin compositions obtained in Examples andComparative Examples. A film carrying a negative image was applied tothe sample piece, and a UV ray was irradiated onto the film at 30 mW/cm²(radiation wavelength=200-450 nm) for 1 minute. The negative film wasthen removed and the uncured area (unexposed area) was dissolved bytoluene and was scraped off with the help of a brush. Subsequently, thesample piece was dried at 60° C. for 30 minutes and was then irradiatedwith a UV ray at 30 mW/cm² (radiation wavelength=200-450 nm) for 10minutes. Using a light microscope at 50× magnification, the resultingplate was observed for how well the thin lines forming projected areasare reproduced and recessed areas are carved. The sample was determinedto be “acceptable” if the negative image was precisely reproducedwithout cracking or chipping of the image (indicated by “G”) and“non-acceptable” if the reproduction was insufficient (indicated by“NG”).

POLYMERIZATION EXAMPLE 1

50 kg of cyclohexane and 265 ml of a cyclohexane solution ofsec-butyllithium (11% by mass) were placed in a pressure vessel equippedwith a stirrer. To this solution, 2.25 kg of p-methylstyrene were addedover a 30-minute period and the polymerization was allowed to proceed at50° C. for 120 minutes. Following the addition of 80 g oftetrahydrofuran, 10.5 kg of butadiene were added over a 60-minute periodand the polymerization was allowed to proceed at 50° C. for 30 minutes.Additional 2.25 kg of p-methylstyrene were added over a 30-minute periodand the polymerization was allowed to proceed at 50° C. for 30 minutes.This gave a reaction mixture containing apoly(p-methylstyrene)-polybutadiene-poly(p-methylstyrene) triblockcopolymer (which is referred to as “block copolymer (I)-1,”hereinafter). The resulting block copolymer (I)-1 had a number averagemolecular weight of 76400, and the amount of p-methylstyrene asdetermined by ¹H-NMR was 30% by mass. It was determined that 40 mol % ofthe butadiene units forming the polybutadiene block were linked by1,2-linkage.

To the resulting reaction mixture containing the block copolymer (I)-1,a hydrogenation catalyst, separately prepared by adding 400 g oftriisopropylaluminum (20% by mass, cyclohexane solution) to 130 g ofnickel octanoate (64% by mass, cyclohexane solution), was added, and thehydrogenation reaction was allowed to proceed at 80° C. in a hydrogenatmosphere of 1 MPa. This gave a hydrogenated product of the polyp-methylstyrene-polybutadiene-poly p-methylstyrene triblock copolymer(The product is referred to as “block copolymer (I)-2,” hereinafter).The resulting block copolymer (I)-2 had a number average molecularweight of 77000, and the amount of p-methylstyrene and the degree ofhydrogenation as determined by ¹H-NMR were 29% by mass and 97%,respectively.

POLYMERIZATION EXAMPLE 2

50 kg of cyclohexane and 130 ml of a cyclohexane solution ofsec-butyllithium (11% by mass) were placed in a pressure vessel equippedwith a stirrer. To this solution, 1.57 kg of p-methylstyrene were addedover a 30-minute period and the polymerization was allowed to proceed at50° C. for 120 minutes. Following the addition of 120 g tetrahydrofuran,12.2 kg of isoprene were added over a 60-minute period and thepolymerization was allowed to proceed at 50° C. for 30 minutes.Additional 1.57 kg of p-methylstyrene were added over a 30-minute periodand the polymerization was allowed to proceed at 50° C. for 30 minutes.This gave a reaction mixture containing apoly(p-methylstyrene)-polyisoprene-poly(p-methylstyrene) triblockcopolymer (which is referred to as “block copolymer (I)-3,”hereinafter). The resulting block copolymer (I)-3 had a number averagemolecular weight of 127000, and the amount of p-methylstyrene asdetermined by ¹H-NMR was 20% by mass. It was determined that 40 mol % ofthe isoprene units forming the isoprene block were linked by 1,2-linkageor 3,4-linkage.

POLYMERIZATION EXAMPLE 3

50 kg of cyclohexane and 265 ml of a cyclohexane solution ofsec-butyllithium (11% by mass) were placed in a pressure vessel equippedwith a stirrer. To this solution, 2.25 kg of styrene were added over a30-minute period and the polymerization was allowed to proceed at 50° C.for 120 minutes. Following the addition of 80 g tetrahydrofuran, 10.5 kgof butadiene were added over a 60-minute period and the polymerizationwas allowed to proceed at 50° C. for 30 minutes. Additional 2.25 kg ofstyrene were added over a 30-minute period and the polymerization wasallowed to proceed at 50° C. for 30 minutes. This gave a reactionmixture containing a polystyrene-polybutadiene-polystyrene triblockcopolymer (which is referred to as “block copolymer 1” hereinafter). Theresulting block copolymer 1 had a number average molecular weight of76400, and the amount of styrene as determined by ¹H-NMR was 30% bymass. It was determined that 40 mol % of the butadiene units forming thepolybutadiene block were linked by 1,2-linkage.

To the resulting reaction mixture containing the block copolymer 1, ahydrogenation catalyst, separately prepared by adding 400 g oftriisopropylaluminum (20% by mass, cyclohexane solution) to 130 g ofnickel octanoate (64% by mass, cyclohexane solution), was added, and thehydrogenation reaction was allowed to proceed at 80° C. in a hydrogenatmosphere of 1 MPa. This gave a hydrogenated product of thepolystyrene-polybutadiene-polystyrene triblock copolymer (The product isreferred to as “block copolymer 2,” hereinafter). The resulting blockcopolymer 2 had a number average molecular weight of 77000, and theamount of styrene and the degree of hydrogenation as determined by¹H-NMR were 29% by mass and 97%, respectively.

POLYMERIZATION EXAMPLE 4

50 kg of cyclohexane and 130 ml of a cyclohexane solution ofsec-butyllithium (11% by mass) were placed in a pressure vessel equippedwith a stirrer. To this solution, 1.57 kg of styrene were added over a30-minute period and the polymerization was allowed to proceed at 50° C.for 30 minutes. Following the addition of 120 g tetrahydrofuran, 12.2 kgof isoprene were added over a 60-minute period and the polymerizationwas allowed to proceed at 50° C. for 30 minutes. Additional 1.57 kg ofstyrene were added over a 30-minute period and the polymerization wasallowed to proceed at 50° C. for 30 minutes. This gave a reactionmixture containing a polystyrene-polyisoprene-polystyrene triblockcopolymer (which is referred to as “block copolymer 3” hereinafter). Theresulting block copolymer 3 had a number average molecular weight of127000, and the amount of styrene as determined by ¹H-NMR was 20% bymass. It was determined that 40 mol % of the isoprene units forming thepolyisoprene block were linked by 1,2-linkage or 3,4-linkage.

EXAMPLES 1 THROUGH 3

(1) The block copolymer (I)-2 or the block copolymer (I)-3,1,9-nonanediol diacrylate, benzophenone, and 2,6-di-t-butyl-p-cresol toserve as a heat polymerization inhibitor were mixed together in thecorresponding proportion (in % by mass) shown in Table 1 below. Thecomponents were kneaded on a kneader at 180° C. for 3 minutes and theresulting curable resin composition was pressed by use of a pressingmachine heated to 180° C. under a pressure of 10 MPa for 3 minutes, tomake a 2 mm-thick sheet.

(2) The sheet obtained in (1) above was evaluated for the shapestability in the manner described above. The results are shown in Table1 below.

(3) The sheet obtained in (1) above was irradiated with a UV ray at 30mW/cm² for 1 minute and was evaluated for the tensile strength at breakand elongation at break in the manner described above. The results areshown in Table 1 below.

(4) A 15 cm×15 cm sample piece was made out of the sheet obtained in (1)above. A film carrying a negative image was applied to one surface ofthe sample piece and a UV ray was irradiated from above the negativefilm at 30 mW/cm². The sample piece was entirely irradiated for 1minute. The reproducibility of the negative image then was evaluated inthe manner described above. The results are shown in Table 1 below.

Comparative Examples 1 Through 3

In each of Comparative Examples 1 through 3, a sheet was prepared in thesame manner as in Examples 1 through 3, except that the block copolymer2 or the block copolymer 3 was used in place of the block copolymer(I)-2 or the block copolymer (I)-3. The sheet was similarly evaluatedfor the shape stability of uncured plate, the tensile strength at break,the elongation at break, and the reproducibility of negative image. Theresults are shown in Table 1 below.

TABLE 1 Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 1 Ex. 2 Ex. 3 Proportion(% mass) Block copolymer (I)-2 86 Block copolymer (I)-3 86 71 Blockcopolymer 2 86 Block copolymer 3 86 71 1,9-nonanediol 10 10 25 10 10 25diacrylate Benzophenone 3 3 3 3 3 3 2,6-di-t-butyl cresol 1 1 1 1 1 1tensile strength at 10 12 8.6 6.8 7.2 5.8 break (MPa) elongation atbreak 440 480 320 220 260 210 (%) Shape stability of G G G G NG NGuncured plate Reproducibility of G G G NG G G negative image

A comparison between Examples 1 through 3 and respective ComparativeExamples 1 through 3 indicates that each of the sheets obtained byirradiating respective sheets of the curable resin compositions of thepresent invention shows a superior tensile strength at break and animproved elongation at break. This suggests that when used as a materialof a flexographic plate, these compositions are each expected to improvethe tensile strength at break and elongation at break of the cured areaof the plate. In addition, the sheets made of the curable resincompositions of Examples 1 through 3 exhibit a better shape stability ofuncured plate as compared to those of respective Comparative Examples 1through 3. This indicates that each of the sheets made of the curableresin compositions of the present invention is expected to offer animproved reproducibility of negative images when actually used in theproduction of flexographic plates (i.e., application of negative film,followed by irradiation with a UV ray, followed by removal of uncuredarea).

EXAMPLES 4 THROUGH 6

(1) The block copolymer (I)-1, the block copolymer (I)-2, or the blockcopolymer (I)-3, a liquid polybutadiene NISSO-PB C-1000 [Nippon SodaCo., Ltd., α,ω-polybutadiene dicarboxylic acid, number average molecularweight=1200-1550, viscosity=10-30 Pa·s (100-300 poise; 45° C.)],1,9-nonanediol diacrylate, benzophenone, and 2,6-di-t-butyl-p-cresol -to serve as a heat polymerization inhibitor were mixed together in thecorresponding proportion shown in Table 1. The components were kneadedon a kneader at 150° C. for 3 minutes and the resulting curable resincomposition was pressed on a press heated to 150° C. under a pressure of10 MPa for 3 minutes, to make a 2 mm-thick sheet.

(2) The sheet obtained in (1) above was evaluated for the shapestability in the manner described above. The results are shown in Table2 below.

(3) The sheet obtained in (1) above was irradiated with a UV ray at 30mW/cm² for 1 minute and was evaluated for the tensile strength at breakand elongation at break in the manner described above. The results areshown in Table 2 below.

(4) A 15 cm long×15 cm wide sample piece was made out of the sheetobtained in (1) above. A film carrying a negative image was applied toone surface of the sample piece and a UV ray was irradiated from abovethe negative film at 30 mW/cm². The sample piece was entirely irradiatedfor 1 minute. The reproducibility of the negative image then wasevaluated in the manner described above. The results are shown in Table2 below.

Comparative Examples 4 Through 6

In each of Comparative Examples 4 through 6, a sheet was prepared in thesame manner as in Examples 4 through 6, except that the block copolymer1, the block copolymer 2, or the block copolymer 3 was used in place ofthe block copolymer (I)-1, the block copolymer (I)-2, or the blockcopolymer (I)-3. The sheet was similarly evaluated for the shapestability of uncured plate, the tensile strength at break, theelongation at break, and the reproducibility of negative image. Theresults are shown in Table 2 below.

TABLE 2 Comp. Comp. Comp. Ex. 4 Ex. 5 Ex. 6 Ex. 4 Ex. 5 Ex. 6 Proportion(% mass) Block copolymer (I)-1 60 Block copolymer (I)-2 60 Blockcopolymer (I)-3 60 Block copolymer 1 60 Block copolymer 2 60 Blockcopolymer 3 60 Liquid polybutadiene 33 33 33 33 33 33 (NISSO-PB C-1000)1,9-nonanediol 5 5 5 5 5 5 diacrylate Benzophenone 1.5 1.5 1.5 1.5 1.51.5 2,6-di-t-butyl cresol 0.5 0.5 0.5 0.5 0.5 0.5 tensile strength at8.1 7.0 7.2 4.8 4.2 4.3 break (MPa) elongation at break 300 380 420 200210 220 (%) Shape stability of G G G NG G NG uncured plateReproducibility of G G G NG NG G negative image

A comparison between Examples 4 through 6 and respective ComparativeExamples 4 through 6 indicates that each of the sheets obtained byirradiating respective sheets of the curable resin compositions of thepresent invention shows a superior tensile strength at break and animproved elongation at break. This suggests that when used as a materialof a flexographic plate, these compositions are each expected to improvethe tensile strength at break and elongation at break of the cured areaof the plate. In addition, the sheets made of the curable resincompositions of Examples 4 through 6 exhibit a better shape stability ofuncured plate as compared to those of respective Comparative Examples 4through 6. This indicates that each of the sheets made of the curableresin compositions of the present invention is expected to offer animproved reproducibility of negative images when actually used in theproduction of flexographic plates (i.e., application of negative film,followed by irradiation with a UV ray, followed by removal of uncuredarea).

INDUSTRIAL APPLICABILITY

As set forth, the flexographic plate material using the curable resincomposition of the present invention as its constituent can be cured toform strong and extendable areas and can be used to make flexographicplates that can form a sharp image plate surface even for a fine image.The flexographic plate material of the present invention is particularlyuseful in printing on cardboards, recycled paper or other paper articleswith rough surfaces.

In addition, the flexographic plate material of the present inventionallows production of flexographic plates that undergo minimumdeformation during storage or transportation of uncured plates (i.e.,flexographic plate material yet to be irradiated with the active energyray), can form a sharp image plate surface by irradiating with theactive energy ray, and can ensure high quality printing in printing oncardboards, recycled paper and other poor quality paper articles havingrough surfaces by being capable of effectively transferring an ink ontosuch paper articles.

1. A cured material, obtained by irradiating a curable resin compositionwith an active energy ray so that a moiety of a polymer block Acontained in the curable resin composition is crosslinked, the curableresin comprising an addition polymerization-based block copolymer (I),an ethylenic unsaturated compound (II), and a photopolymerizationinitiator (III), wherein: the addition polymerization-based blockcopolymer (I) is selected from block copolymers comprising at least onepolymer block A and at least one polymer block B, and the hydrogenatedproducts thereof; the polymer block A comprises an aromatic vinylcompound unit containing at least 10% by mass of an alkylstyrene-derivedstructural unit (a) in which at least one alkyl group having 1 to 8carbon atoms is bound to a benzene ring; and the polymer block Bcomprises a conjugated diene compound unit.
 2. The cured materialcomposition according to claim 1, further comprising a softener (IV). 3.The cured material composition according to claim 1, wherein thestructural unit (a) in which at least one alkyl group having 1 to 8carbon atoms is bound to a benzene ring is a p-methylstyrene unit.
 4. Aflexographic plate material, comprising the cured material according toclaim 1 as a constituent.
 5. The cured material composition according toclaim 1, wherein the structural unit (a) in which at least one alkylgroup having 1 to 8 carbon atoms is bound to a benzene ring is ap-ethylstyrene unit.